This invention relates to a lithography device using a source of extreme ultraviolet radiation and multi-layered mirrors provided to reflect this extreme ultraviolet radiation that is also called xe2x80x9cEUV radiationxe2x80x9d or xe2x80x9cX-UV radiationxe2x80x9d.
The wavelength of such radiation is within the range extending from 8 nm to 25 nm.
The invention is applicable most particularly to the manufacture of integrated circuits with a very high degree of integration, the use of EUV radiation enabling one to reduce the etch spacing of such circuits.
In the main, two techniques are known for producing intense EUV radiation. Both of them rely on the collection of photons produced by the microscopic process of spontaneous emission by a hot plasma of low density which is generated by means of a laser.
The first technique uses a jet of xenon irradiated by a YAG laser, the power of which is close to 1 kW. In effect, when the nature of the gas and the conditions for expansion into the vacuum are well chosen, clusters are naturally created in the jet through multi-body interactions. These clusters are macro-particles which can contain up to a million atoms and have a density which is sufficiently high (about one tenth of the density of the solid) to absorb the laser beam and thereby heat the atoms of the surrounding gas which can then emit photons through fluorescence.
The second technique uses the corona of a plasma of high atomic number, obtained by interaction of a laser beam, which comes from a KrF laser, the intensity of which is close to 1012 W/cm2, and a solid target of great thickness (at least 20 xcexcm).
The laser beam is focused on one face of this target, called the xe2x80x9cfront facexe2x80x9d and one uses the EUV radiation emitted by this front face and generated by interaction of the laser beam and the material of the target.
If the first or the second technique is used, the EUV radiation obtained comprises a continuous energy spectrum and with strong emission lines.
The UEV radiation sources which the first and second techniques use have the following disadvantages.
These sources have an isotropic emission which therefore has a large angular divergence, and the emitted EUV radiation spectrum includes lines of low spectral width.
It is then necessary to associate with each source, complicated optical collection means which enable one to recover the maximum from the wide angular field of emission from the source.
These optical means formed by multi-layered mirrors, must be produced in such a way that their spectral responses are centered on the emission line chosen for the exposure of a sample, restricting as much as possible the loss of intensity due to multiple reflections on the multi-layered mirrors.
A known example of a lithography device using EUV radiation, the wavelengths of which are situated, for example, close to the range from 10 nm to 14 nm is diagrammatically shown in FIGS. 1 and 2. Such a device is also called an xe2x80x9cEUV lithography devicexe2x80x9d.
This known device is intended to expose a sample E. Generally, this is a semi-conductor substrate 2 (for example made of silicon) onto which a layer of photosensitive resin (xe2x80x9ca photo-resist layerxe2x80x9d) 3 has been deposited and it is desired to expose this layer in accordance with a specified pattern.
After exposure of the layer 3, it is developed and the substrate 2 can then be etched in accordance with the pattern.
The device in FIGS. 1 and 2 includes
a support 4 for the sample,
a mask 5 comprising the specified pattern in an enlarged form
a source 6 of radiation in the extreme ultraviolet range (FIG. 2),
optical means 7 for the collection and the transmission of the radiation to the mask 5, the latter providing an image of the pattern in enlarged form, and
optical means 8 for reducing this image and projecting the reduced image onto the layer 3 of photosensitive resin (chosen in such a fashion that it is sensitive to the incident radiation).
The known source 6 of EUV radiation comprises means of forming a jet J of clusters of xenon. Only the nozzle 9 which includes these formation means is represented in FIG. 2.
The source also comprises a laser (not shown), the beam of which F is focused onto a point S of the jet J by the optical means of focusing 10. The interaction of this beam F and the xenon clusters generate the EUV radiation R.
The point S is visible in FIG. 1 (but not the nozzle nor the jet of xenon clusters).
Among the optical means 7 of the device for collection and transmission, there is an optical collector 11 provided with a central opening 12 to allow the focused laser beam F to pass.
This optical collector 11 is positioned facing the jet of xenon clusters and is intended to collect a part of the EUV radiation emitted by the xenon clusters and to transmit this collected radiation 13 toward other optical components that also form a part of the optical means 7 for collection and for transmission.
These optical means 7 for collection and for transmission, the mask 5, which is used in reflection, and the optical means 8 for reduction and for projection are multi-layered mirrors 14 which selectively reflect the EUV radiation and are designed in such a way that their spectral responses are centered on the wavelength chosen for exposure of the layer of photosensitive resin 3.
It should be made clear that the pattern, in accordance with which one wishes to etch the sample, is formed on the multi-layered mirror corresponding to the mask 5, with an enlargement factor suited to the optical means for reduction and for projection, and this multi-layered mirror is coated, except for the pattern, with a layer (not shown) which is capable of absorbing the incident EUV radiation.
Within the wavelength range of EUV radiation, the spectral resolution xcex94xcex/xcex of the mirrors is about 4%.
The breadth of the spectral range usable for exposure is obtained by the convolution of the spectral breadth of the EUV radiation and this spectral resolution.
The known multi-layered mirrors to which we will return subsequently and which are used in the lithography device shown in FIGS. 1 and 2, have, in particular, the following disadvantage: their spectral band, which is centered on the wavelength chosen for the exposure, is narrow.
The result is a reduction in the efficiency of the lithography device.
These EUV multi-layered mirrors also have the disadvantage of deforming when they are exposed to a high thermal flux coming from the source of EUV radiation for the device.
One aim of the invention is to propose an EUV lithography device that is much more efficient than the known devices considered to be the most highly efficient.
The device which is the subject of the invention comprises a source of EUV radiation which is anisotropic. This EUV radiation is emitted through the back face of a solid target of suitable thickness on the front face of which a laser beam is focused.
Such an anisotropic source enables one to increase the effective portion of the EUV radiation beam and to simplify the collection of this radiation.
Furthermore, the device which is the subject of the invention comprises multi-layered mirrors capable of reflecting the generated EUV radiation, each layered mirror having a spectral band (also called xe2x80x9cspectral widthxe2x80x9d or xe2x80x9cbandwidthxe2x80x9d) greater than that of the known multi-layered mirrors mentioned above.
The source used in the invention, the emission spectrum of which is closer to black body over a broad spectral range, and the multi-layered mirrors with a broad spectral bandwidth, also used in the invention, work together to lead to a device capable of supplying the sample, which one wishes to expose with EUV radiation which is more intense than in the prior art.
Another aim of the invention is to minimize thermal deformation of the multi-layered mirrors which are used in the invention when these multi-layered mirrors are exposed to the intense flux of EUV radiation.
To put it precisely, the subject of this invention is a lithography device comprising:
a support for a sample intended to be exposed in accordance with a specified pattern,
a mask comprising the specified pattern in an enlarged form,
a source of radiation in the extreme ultraviolet range,
optical means for the collection and for the transmission of the radiation to the mask, the latter supplying an image of the pattern in enlarged form, and
optical means for the reduction of this image and for the projection of the reduced image onto the sample,
the mask, the optical means for collection and transmission and the optical means for reduction and projection comprising multi-layered mirrors, each multi-layered mirror comprising a substrate and, on this substrate, a stack of layers of a first material and of layers of a second material which alternate with the layers of the first material, this first material having an atomic number greater than that of the second material, the first and second layers co-operating to reflect the extreme ultraviolet radiation, the stack having a free surface onto which the radiation to be reflected arrives,
this device being characterized in that the source comprises at least one solid target, having first and second faces, this target being capable of emitting, in an anisotropic way, a part of the extreme ultraviolet radiation from the second face of this target, in that the optical means for collection and for transmission are provided in order to transmit, to the mask, the part of the extreme ultraviolet radiation coming from the second face of the target of the source and in that the thickness of pairs of adjacent layers, in the stack of layers that each mirror comprises, is a monotonic function of the depth in the stack, this depth being counted from the free surface of the stack.
By a xe2x80x9cmonotonic functionxe2x80x9d one understands a function which is either increasing or decreasing.
According to a preferred embodiment of the device which is a subject of the invention, the target contains a material which is capable of emitting the extreme ultraviolet radiation by interaction with the laser beam and the thickness of the target is within a range extending from about 0.05 xcexcm to about 5 xcexcm.
Preferably, the target contains a material which is capable of emitting the extreme ultraviolet radiation through interaction with the laser beam and which has an atomic number belonging to the group of atomic numbers ranging from 28 to 92.
According to one particular embodiment of the device which is a subject of the invention, this device comprises a plurality of targets which are made integral one with another, the device additionally comprising means of displacing this plurality of targets so that these targets successively receive the laser beam.
The device may additionally comprise support means to which the targets are fixed and which are capable of allowing the laser beam to pass in the direction of these targets, the means of displacement being provided in order to displace these means of support and hence the targets.
These means of support can be capable of absorbing radiation emitted by the first face of each target which receives the laser beam and of re-emitting this radiation towards this target.
According to a first particular embodiment of the device which is a subject of the invention, the means of support comprise an opening facing each target, this opening being defined by two sidewalls, substantially parallel to one another and perpendicular to this target.
According to a second particular embodiment, the means of support comprise an opening facing each target, this opening being defined by two sidewalls which become further apart from one another as they go towards the target.
According to a particular embodiment of the invention, the device additionally comprises auxiliary fixed means which are capable of allowing the laser beam to pass in the direction of the target, of absorbing the radiation emitted by the first face of this target and of re-emitting this radiation towards this target.
According to a preferred embodiment of the invention, the stack which each multi-layered mirror comprises, is subdivided into assemblies of at least one pair of first and second layers and the thickness of these assemblies is a monotonic function of the depth in the stack, this depth being counted from the free surface of the stack.
According to a particular embodiment of the invention, the increases in thickness of these assemblies form an arithmetic progression.
Preferably, the first and second layers of each assembly have approximately the same thickness.
By way of example, the first and second layers may be respectively molybdenum and beryllium or molybdenum and silicon.
The substrate may, for example, be made of silicon or germanium.
Preferably, the thickness of the substrate is within the range extending from about 5 mm to about 40 mm and the thickness of the stack is about 1 xcexcm.
According to a preferred embodiment of the invention, each multi-layered mirror is fitted with means of cooling this multi-layered mirror in order to reduce its deformation when it is illuminated by the EUV radiation.
Preferably, these cooling means are provided in order to cool the mirror to a temperature roughly equal to 100 K.
For example the means of cooling the mirror are liquid helium, Freon, liquid nitrogen or a cooling fluid which is a heat transfer fluid at a low temperature close to 0 K.
The sample that it is desired to expose may comprise a semi-conductor substrate on which a layer of photo-sensitive resin is deposited and is intended to be exposed in accordance with the specified pattern.